FR2757961A1 - METHOD FOR MANUFACTURING MICROSTRUCTURES BY MULTILAYER CONFORMATION OF A PHOTOSENSITIVE RESIN AND MICROSTRUCTURES THUS OBTAINED - Google Patents

Abstract

A method of manufacturing a three-dimensional microstructure comprising: a) creating a sacrificial coating (5) on a wafer-support (4); b) spreading 150 mum and 700 mum of negative photoresist (6, 21), sensitive to ultraviolet radiation, all at once; c) heat; d) effecting UV illumination through a mask (7, 12); e) perform annealing; f) reproduce at least once steps b) to e) using, if necessary according to the desired contour for the structuring of the new layer (8, 23, 27) of photoresist, a mask (9, 24, 28) different for step d); g) developing in one or more steps, and h) separating the microstructure from the support plate (4) by dissolving the sacrificial coating (5). Microstructures obtained by said method, such as a toothed wheel surmounted by a pinion, a micromould to obtain it, and a thermal flux microsensor.

Description

PROCESS FOR PRODUCING MICROSTRUCTURES BY

  MULTILAYER CONFORMATION OF A RESIN

  PHOTOSENSITIVE AND MICROSTRUCTURES SO OBTAINED

  The present invention relates to a method of manufacturing three-dimensional microstructures by conformation of at least two thick layers of an ultraviolet light-sensitive resin. The invention also relates to the microstructures thus obtained replicably with a very good definition for an advantageous unit cost, and having a sufficient thickness of the order of a millimeter to withstand mechanical stresses when they are used either directly, for example as components in micromechanics, or indirectly as sacrificial positive structures for the manufacture of metal micro-molds. The production of three-dimensional microstructures took off in the 1980s with the method designed by W. Ehrfeld of the Karlsruhe Nuclear Research Center (Germany) and known by the acronym LIGA (Lithography, Galvanisierung, Abformung). In principle, the LIGA technique consists of: 1) depositing on a substrate a layer of 10 to 1000 μm of a positive photoresist (generally polymethylmethacrylate PMMA); 2) to perform X-irradiation through a mask using a synchrotron; 3) to develop, that is to say to remove by physical or chemical means portions of photoresist depolymerized. The LIGA process, and derived processes still employing X irradiation, provide microstructures having a high aspect ratio, a value defined as the quotient of the height by the width of the line or trench. finer and constituting a criterion for assessing the verticality of the walls. The quality of the microstructures obtained does not lend itself to criticism, but the fact of having to implement expensive equipment (synchrotron) makes this technique little compatible with a mass production of microstructures to have a low unit cost. To reduce costs, research has turned to the so-called "LIGA poor" process in which the sensitization of a photoresist layer is produced by the ultraviolet radiation of a conventional aligner. The maximum thickness of the microstructures currently obtained with this method is between 80 μm and 200 μm. To increase this thickness, it is proposed in patent DE 4,400,315 to repeat the process several times by making between each layer of photoresist a metal interlayer anchoring deposit to reach a total thickness still limited to 400 μm, further having a more complex development stage due to metal deposits. In addition, it will be observed that the number of steps and the presence of intermediate metal deposits contribute to substantially increase the cost.

  and to have a negative impact on the quality of the walls.

  The present invention therefore aims to overcome the disadvantages of this prior art by providing a process for structuring a negative photoresist by ultraviolet radiation in which the particular choice of photoresist surprisingly allows to structure at least two successive thick layers, without intermediate metal deposit and thus to obtain at low cost unit 3 complex three-dimensional microstructures with a thickness of at least up to millimeter, while having an aspect ratio

  higher than those obtained until now by UV structuring.

  To this end, the subject of the invention is a method for manufacturing a three-dimensional microstructure, characterized in that it comprises the steps of: a) creating on a support-plate a sacrificial coating; b) spreading at one time a negative photoresist layer, sensitive to ultraviolet radiation, with a thickness of between 150 gm and 700 μm, said photoresist being formed by a lacquer containing a polyfunctional epoxy composition and a photoinitiator chosen from triarylsulfonium; c) heating said layer between 90 and 95 C for a duration depending on the deposited thickness; d) perform through a mask corresponding to the desired imprint a UV illumination of 200 to 1'000 mJ.cm-2, measured at a wavelength of 365 nm depending on the thickness of the layer; e) annealing said layer to cause polymerization; f) reproducing at least once steps b) to e) using, if necessary depending on the desired contour for structuring the new photoresist layer, a different mask for step d); g) proceed, on one or more occasions, to the development by dissolving the unexposed photoresist by means of a solvent chosen from GBL (gammabutyrolactone) and PGMEA (propylene glycol methyl ethyl acetate), and (h) separating from the platelet-support microstructure obtained in step g) by dissolving the sacrificial coating. The annealing carried out in step e) to cause the polymerization of the zones illuminated by the UV radiation is preferably carried out between 90 ° C. and 95 ° C. for 15 min. In the case where the new layer of photoresist to be illuminated (step f) has a contour which is not totally inscribed in the contour of the lower layer, the deposition of the new photoresist layer is preceded by a step i) consisting of performing a metal deposition (eg Cr / Cu), obstructing the UV rays for the lower layer fraction whose polymerization is not desired. The additional step i) is in particular necessary when the microstructure to be obtained has a cavity. In this case, it will also be necessary to repeat step g) of development after step h) of releasing the structure. In the case where the microstructure obtained after step g) is to serve as a template for the manufacture of a metal micromold, is carried out before step h) an additional step ii) consisting of a primary metallization on any the surface of this template, then the entire assembly is covered completely by successive electrolytic deposits, so as to form a block which will subsequently be machined to be adapted to a

  injection machine for molten plastic materials. After releasing the block from the substrate (step h) it is still necessary during an additional step iii) to remove the template, by physical or chemical means.

  The support wafer used in the first step is for example formed by a wafer of silicon, glass or ceramic on which was deposited by evaporation a sacrificial aluminum layer of about 100 nm. In this case, to release the structure in the last step an alkaline attack is carried out. Equivalently, a composite plastic material can be used providing both good adhesion between the carrier plate and the photoresist deposits, and easy release of the microstructure at the end of the process. The masks used (steps d and f) to delimit on the photoresist layer the surface to receive the UV radiation are made with a very high resolution by the methods used in the microelectronics industry. The beaches to be opaque to UV radiation have for example a chromium metal coating. The photoresist used in the process according to the invention for carrying out a multilayer structuring is a cheap product preferably prepared from an octofunctional epoxy resin available from Shell Chemical under the reference SU-8 and from a photoinitiator chosen from the salts triarylsulfonium such as those described in US Pat. No. 4,058,401.

  revealed the most appropriate is gammabutyrolactone (GBL).

  The subject of the present invention is also the three-dimensional microstructures obtained, characterized by a remarkably high aspect ratio compared to that of the structures obtained hitherto by UV illumination, especially in view of the thickness of said structures. The tests carried out have shown that this ratio could be of the order of 20 for the structures of line and of the order of 15 for the

trench structures.

  The characteristics and advantages of the invention

  will be better understood by reading the description

  The following detailed description of embodiments of microstructures, with reference to the accompanying drawings in which: - Figure 1 is a perspective representation of a first embodiment illustrated by a wheel

toothed topped with a pinion.

  FIGS. 2 to 5 are diametral sectional views of the various steps in the fabrication of the microstructure shown in FIG. 1; Figures 6 and 7 show the additional steps for obtaining a metal micromold from the jig of Figure 5; FIG. 8 is a partially cutaway perspective view of a second embodiment illustrated by a heat flux sensor, and FIGS. 9 to 15 are diagrammatic sectional views along line XX of FIG. 8 of the various steps of FIG. The following examples correspond to three microstructures obtained by the method according to the invention: Example 1: Manufacture of a toothed wheel surmounted by a pianon FIG. 1 represents a gear wheel 1 surmounted by a pinion 2, the assembly being traversed through and through a hole of axis 3. The height of the wheel is 0.23 mm and that of the pinion of 0.45 mm thus representing a height total of 0.68 mm with a tolerance of 10 im. The respective diameters are 3.037 mm and 0.85 mm with tolerances of 1 gm. Thus, the gear module can be up to 0.016 mm, that is to say well below the minimum that can currently be used by watchmakers when the parts are obtained by molding in micro-machined molds in a mold. metal plate by the well-known technique of wire EDM. FIG. 2 shows the state of the product

  obtained after the first three steps of the process.

  A 0.38 mm thick and 7.5 cm diameter, non-oxidized, carefully cleaned and dehydrated silicon wafer 4 is first covered with a sacrificial coating 5 by aluminum evaporation. This coating typically has a thickness of the order of 100 nm. Then, a thick layer 6 of photoresist is spread out. The dynamic viscosity of the composition chosen is of the order of 15 Pa-s and allows a conventional spinning spread, for example by means of a Karl Sss Gyrset apparatus up to the desired thickness, ie 230 μm. The deposited layer is then heated to 95 ° C to remove the solvent which maintained the desired viscosity resistance. Figure 3 corresponds to the illumination step of the photoresist. For this purpose, a mask 7 having areas transparent to UV rays and opaque zones formed by chromium deposits is placed above the surface of the photoresist. These chromium deposits follow on the one hand the outer contour la of the toothing of the wheel 1, on the other hand the inner contour 3a of the axis hole 3. The same support forming the mask may comprise a large number of corresponding zones. as many microstructures can be manufactured in a single batch, all areas being obtained with a very high resolution photolithography contour, technique15 well known in the microelectronics industry. The photoresist layer 6 is then sensitized by UV illumination by means of an aligner having its intensity peaks at 365 nm and 410 nm. Finally, an annealing is carried out for 25 minutes at 95 ° C. which will cause the polymerization of the photoresist in the zones 6b which have received the UV radiation. This polymerized portion becomes insensitive to a large majority of solvents. On the other hand, the non-illuminated zones 6a can subsequently be dissolved by a solvent. In FIG. 4, the structuring of a second layer of photoresist 8, thick of approximately 450 μm, is represented.

  by means of a second mask 9 whose chromium metal deposits 2a, 3a delimit the contour of the zones transparent to the UV rays corresponding to the pinion 30 2 and its pinhole 3.

  After annealing, this second layer of photoresist is structured in zones 8a which will be soluble in the solvent and polymerized zones 8b forming the pinion 2

and its axis hole 3.

  The uncured photoresist portions 6a, 8a are then removed to obtain the microstructure shown in FIG. 5, always integral with the support plate 4, and formed of the two structured levels 6b, 8b. For this purpose, we use as

  solvent propylene glycol methyl ether acetate (PGMEA).

  In a last step (not shown), the microstructure is released by attacking the attachment layer

of aluminum with K OH (40%).

  EXAMPLE 2 Manufacture of a Metal Micro-Mold From the microstructure obtained in Example 1, but before releasing it from the support-plate 4, a metal coating 10 is made, as shown in FIG. the entire outer envelope, intended to hang the metal block 11 formed by successive electrolytic deposits. The initial metallization is for example carried out by evaporation of nickel for

  form a coating having 30 to 100 nm. The electrolytic deposition steps are carried out from a bath also containing nickel or an alloy incorporating it, according to the techniques known in this field.

  Once the block 11 has been formed, it is released from the substrate, as indicated in Example 1 (FIG. 7), and then the positive structure, which corresponds to the microstructure shown in FIG. 1, is eliminated. For this purpose, the

  polymerized photoresist can be dissolved by N-methyl-

  pyrrolidone (NMP) or removed by plasma etching.

  The block 11 is finally machined, individually or with the batch formed on the support plate, so as to be adapted to a machine for injection of molten plastic materials. According to known techniques in this field, the openings 12, which were initially oriented towards the support plate, are closed by an injection hood through which feed channels of said molten plastic materials. This cover may also include a second indentation also

  obtained according to the method described above.

  Example 3 Manufacture of a Thermal Flow Microsensor With reference to FIG. 8, it can be seen that a heat flux microsensor is formed by a cover 20 resting on a base 16 provided with an opening 17 for the admission of the flow, an outlet opening 18 of the flow and a set of differential resistors 19a, 19b, 19c arranged, in the example shown, on the base 16,10 between the openings 17 and 18 and having external portions connectable to a measuring device. The assembly is covered by a cover 20 forming a channel between the openings 17, 18. Such a microsensor has very small dimensions of the order of 2 × 1 × 1 mm, and its most common fabrication is to make a microphone machining of two silicon wafers which should then be assembled. The current method therefore has the disadvantage of leading to a relatively expensive product, which does not however have all the qualities required in the application envisaged because of the excessive thermal conductivity of silicon (about

W / (m.K.)).

  On the contrary, the method according to the invention makes it possible to obtain an inexpensive thermal flux microsensor that does not require any assembly, and furthermore has little heat loss through the substrate because of the

  low thermal conductivity (about 0.1 W / (m.K.)) of the polymer used as photoresist. This microsensor is made by structuring three layers of photoresist corresponding respectively to the base 16, to the vertical walls 20a and to the closure bottom 20b of the cover 20.

  The various phases of construction of this microsensor are explained in more detail hereinafter with reference to FIGS. 9 to 15. As shown in FIG. 9, on a glass substrate 14 having a sacrificial coating 15, it was deposited by spin a layer 21 of the same photoresist as that used in Example 1, over a thickness of 150 μm, heated at 95 ° C. for about 60 minutes, and then disposed above its surface a mask 22 whose zones 22a coated with The ultraviolet radiation-resistant chromium is located outside the perimeter of the base 16 and above the apertures 17, 18. After UV illumination of about 300 mJ / cm 2 and annealing at about 95 ° C for about 15 minutes. minutes, the polymerization of the zones 21b, that is to say of the entire base 16 is obtained with the exception of the photoresist 21a present in the openings 17, 18 and at the periphery of the base

  16 which will be eliminated later.

  In FIG. 10, the step of depositing resistors 19a, 19b, 19c is shown. This deposition is performed by well-known photolithography techniques and will not be described further. These resistors consist, for example, of a deposit of 100 nm of chromium and

nm of gold.

  FIG. 11 corresponds to the structuring of the walls 20a of the cover 20 according to the same process as that described for FIG. 9. The deposited photoresist layer 23 has a thickness of 300 μm and the mask 24 has chromed zones 24a which delimit the perimeter walls. After UV illumination and annealing, the zones 23b, which extend the zones 21b polymerize and the resistors 19a, 19b,

  19c are embedded in the unpolymerized photoresist 23a.

  FIG. 12 shows an intermediate step of protecting the zone 23a by a metal deposit 25 covering the entire opening of the cover 20. As for the resistors, this deposit is made by well-known photolithography techniques and will not be not described further. It is constituted for example

  by depositing 50 nm of chromium and 300 nm of copper.

  FIG. 13 shows the structuring of a third layer 27 of 150 μm of photoresist by means of a third mask 28, the chromium-coated areas 28a delimiting a rectangular contour corresponding to the bottom.

a hood 20.

  FIG. 14 represents the first phase of development which reveals the microsensor, still integral with the support plate 14 and the sacrificial coating 15, and trapping the fractions

  21a and 23a unpolymerized photoresist.

  As shown in Figure 15, after full release of the structure a second phase of development

  allows to eliminate the fractions 21a and 23a.

  The development and elimination of the sacrificial coating is carried out as indicated in Example 1.

  For development, a PGMEA bath at 50 ° C is preferably used for 15 to 30 minutes, followed by isopropanol rinsing.

  The process which has just been described is not limited to obtaining microstructures given by way of examples and the person skilled in the art is able to conceive other

  constructions without departing from the scope of the invention.

Claims (9)

  1. A method of manufacturing a three-dimensional microstructure, characterized in that it comprises the steps of: a) creating on a support plate (4) a sacrificial coating (5); b) spreading at one time a layer (6, 21) of negative photoresist, sensitive to ultraviolet radiation, having a thickness of between 150 gm and 700 gm, said photoresist being formed by a lacquer containing a polyfunctional epoxy composition and a photoinitiator selected from triarylsulfonium salts; c) heating said layer (6, 21) between 90 and 95 C for a duration depending on the deposited thickness; d) performing through a mask (7, 12) corresponding to the desired imprint a UV illumination of 1'000 mJ.cm-2 measured at a wavelength of 365 nm depending on the thickness of the layer; e) annealing said layer (6, 21) to cause polymerization; f) reproducing at least once steps b) to e) using, if necessary depending on the desired contour for structuring the new layer (8, 23, 27) of photoresist, a mask (9, 24, 28) different for step d); g) proceed in one or more steps by dissolving the unexposed photoresist using a solvent selected from GBL (gammabutyrolactone) and PGMEA (propylene glycol methyl ethyl acetate), and h) separating from the support plate (4) the microstructure obtained in step g) by dissolving the sacrificial coating (5). 2. Method according to claim 1, characterized in that the annealing of step e) is preferably carried out   between 90 C and 95 C for 15 to 30 minutes.   3. Method according to claim 1, characterized in that the plaquettesupport is chosen from   silicon wafers of glass or ceramic.   4. Method according to claim 1, characterized in that the sacrificial bonding coating is selected from metals, such as aluminum, and materials. composite plastics.   5. Process according to claims 1 and 4,   characterized in that the chemical agent for releasing the microstructure of the substrate is 40% KOH for   a sacrificial aluminum coating.   6. Method according to claim 1, characterized in that it further comprises, before one of the steps f), an additional step i) of performing a metal deposit (25) for protecting a UV radiation a   determined area of a low layer of photoresist.   7. Method according to claim 1, characterized in that it further comprises, before the release of the structure of the wafer-support an additional step ii) of covering the microstructure obtained at the end of step g) of a primary metallization then to form a metal block by successive electrolytic deposits, and in a step iii) following step h) to be eliminated by physical or chemical means said   microstructure to form a micromould.   Microstructure obtained according to claim 1, characterized in that the first layer of photoresist is structured in the form of a toothed wheel (1) and the second   sprocket-like photoresist layer (2).   9. Microstructure obtained according to claim 6, characterized in that the structuring of three layers of photoresist allows to obtain a hollow microstructure provided with two openings (17, 18) positioned on either side of three resistors (19a, 19b). , 19c) for form a thermal microsensor.